In the bidirectional telephone circuit or speaker-phone (a telephone having a loudspeaker) which is now used most widely, it is necessary to use a device for suppressing only one of two coexisting signals to prevent sidetalk or sidetone. Here, "sidetalk or sidetone" is defined as a kind of acoustic feedback which is so weak that it does not cause howling.
Conventional signal control circuits for preventing sidetalk (that is, anti-sidetone circuits) have been designed to prevent sidetalk by balancing an impedance bridge as shown in FIG. 1, or alternatively, by utilizing a booster circuit employing an antisidetalk coil as shown in FIG. 2. In FIG. 1, numeral 1 designates a first signal source connected through a transformer 3 to the junction point of resistors 5 and 7 and to that of resistors 6 and 8. Numeral 2 designates a second signal source having an internal impedance Z and connected across the resistor 5 through a transformer 4. Assuming that the turn ratio of the transformer 4 is 1:1, if the ratio of the combined impedance of the resistance of the resistor 5 and the internal impedance Z of the second signal source 2 to the resistance of the resistor 6 is made equal to the ratio of the resistance of the resistor 7 to that of the resistor 8, then there appears no output signal of the first signal source 1 at an output terminal 9, but only an output signal of the second signal source 2 is delivered to the output terminal 9. However, if the internal impedance Z of the second signal source 2 varies, the bridge will be out of balance, thereby causing the output signal of the first signal source 1 to appear at the output terminal 9 and hence deteriorating its antisidetalk characteristic.
Referring now to FIG. 2, numeral 11 designates a first signal source, 12 a second signal source having an internal impedance Z, 16 output terminals, 17 a resistor, and 18 a coupling capacitor for blocking a d-c voltage, which elements are connected, as shown in FIG. 2, to windings 13, 14 and 15 wound on the same iron core. The appearance of an output signal of the first signal source 11 at the output terminals 16 may be prevented by cancelling a current flowing into the output terminals 16 with an electromotive force induced in the winding 15 and by making the ratio of the resistance of the resistor 17 to the internal impedance Z of the second signal source 12 take a constant value and by suitably selecting the number of turns of the windings 13, 14 and 15. However, also in this case, the balance will be lost by variations in the impedance Z with the resultant deterioration of its antisidetalk characteristic. Assuming now that in FIGS. 1 and 2 each of the signal sources 2 and 12 represents a wire telephone network, each of the signal sources 1 and 11 represents a wire telephone transmitter, and each of the output terminals 9 and 16 represents a receiver, it is necessary to prevent sidetalk from being caused by the appearance of a signal, which has been sent into the circuit by the signal sources 1 and 11, at the output terminals 9 and 16, respectively. With the conventional circuits as shown in FIGS. 1 and 2, it has been impossible to always surely prevent the occurrence of sidetalk, since the impedance of the wire telephone network varies depending on the manner of the connection of telephone circuits in the network.